An exhaust purifying mechanism that purifies exhaust gas by trapping particulate matter (PM) in exhaust gas with a filter located in an exhaust passage has been employed for an internal combustion engine such as a vehicle diesel engine. In such an exhaust purifying mechanism, the filter needs to be regenerated by removing particulate matter accumulated in the filter before the filter is clogged by accumulation of trapped particulate matter.
Japanese Laid-Open Patent Publication No. 2003-20930 discloses an exhaust purifying apparatus that eliminates particulate matter in the filter. In the exhaust purifying apparatus of the above publication, the filter supports a catalyst that promotes oxidization of particulate matter, and fuel is added to exhaust gas that flows into the filter. The particulate matter trapped in the filter is oxidized (burned) by adding fuel, and the filter is thus regenerated.
When adding fuel as described above, if fuel keeps being added to the exhaust purifying mechanism, the fuel continues to be burned at the upstream section of the mechanism. Thus, exhaust gas having high temperature is continuously sent to the downstream section of the mechanism. Therefore, the temperature of the exhaust purifying mechanism tends to increase toward the downstream end. Thus, particulate matter is likely to remain at the upstream section of the mechanism. That is, particulate matter tends to partially remain unburned.
On the other hand, when intermittently adding fuel to exhaust gas, exhaust gas having high temperature is prevented from being continuously sent to the downstream section of the exhaust purifying mechanism. This suppresses the temperature distribution of the exhaust purifying mechanism from being uneven. Therefore, the amount of particulate matter that partially remains in the exhaust purifying mechanism is reduced.
Although the intermittent fuel addition suppresses particulate matter from partially remaining in the exhaust purifying mechanism, oxidization of particulate matter is promoted as compared to a case where the continuous fuel addition is performed. Therefore, if the intermittent fuel addition is not appropriately executed, the temperature of the exhaust purifying mechanism may be excessively increased.